The present invention relates to a pneumatic tire, more particularly an improved tread pattern which is capable of improving the steering stability, tire uniformity and the like without increasing a tread pattern noise.
In the pneumatic tires provided in the tread portion with blocks, lugs and the like (hereinafter tread elements) arranged circumferentially at a constant pitch, a large pattern noise at a specific frequency is liable to occur during running.
In order to reduce such a pattern noise at a specific frequency, there have been proposed, for example in JP-A-2-127103, the so-called variable pitching methods, which disperse the noise into a wide frequency range by, for example, using plural kinds of blocks having different circumferential lengths, or changing the circumferential widths of axial grooves, and thus prevents a resonance. In those variable pitching methods, in order to derive a noise reducing effect effectively therefrom, the maximum pitch ratio, that is, the ratio (b/a) of the maximum length (b) to minimum length (a) of the tread elements must be set at a large value. Accordingly, the rigidity of the tread elements becomes inevitably uneven. As a result, the steering stability is decreased, and the tractive force variation increases and the tire uniformity is deteriorated.
It is therefore, an object of the present invention to provide a pneumatic tire in which the pattern noise is effectively improved without decreasing the steering stability, tire uniformity and the like.
According to one aspect of the present invention, a pneumatic tire comprises
rows of circumferentially arranged tread elements,
the tread elements in each row having plural kinds of different circumferential lengths,
axial grooves circumferentially dividing the tread elements in said each row having plural kinds of different circumferential widths, the circumferential widths of each such axial grooves being generally constant along the length thereof.
the circumferential widths of the axial grooves and the circumferential lengths of the tread elements in each row satisfying
1.2xe2x89xa0xe2x89xa6GR/BRxe2x89xa0xe2x89xa61.5
where
BR is the length maximum ratio defined by the ratio (Bll/Bss) of the maximum circumferential length (Bll) to the minimum circumferential length (Bss), and
GR is the width maximum ratio defined by the ratio (Gll/Gss) of the maximum circumferential width (Gll) to the minimum circumferential width (Gss).
In each row, preferably, the number of the different circumferential lengths of the tread elements is equal to the number of the different circumferential widths of the axial grooves.
As to the sequence of the tread elements in each row, preferably, the circumferential lengths are arranged in the ascending and descending order.
Further, the sequence of the axial grooves is such that each of the circumferential widths is adjacent to one of the circumferential lengths having the same order as the width, whereby the circumferential widths are arranged in the ascending and descending order. In other words, the circumferential widths are respectively located on the same side of the circumferential lengths of the same order, wherein the xe2x80x9csame sidexe2x80x9d is in the circumferential direction, and means, for example, the front side or rear side.